I had my period and it lasted 2 days longer than usual. four days later started my period again. not on birth control, and i know i am not pregnant?

Hormone: This sounds like a hormone imbalance or breakthrough bleeding. Sometimes this occurs because estrogen and Progesterone (your 2 female hormones produced in ovary) are not in the right balance to each other. Sometimes it's due to pituitary or thyroid issues. And lasly, sometimes due to weight (too thin/heavy). If bothers you can consider birth control pills to help regulate. Best of luck.